This invention relates generally to the field of light emitting diodes (LEDs). More particularly, this invention relates to organic light emitting diodes which employ an electron-injecting cathode comprising a thin layer of an oxide of a low work function metal. More specifically, the present invention relates to LEDs which comprise: (a) a hole-injecting anode layer; (b) an electron-injecting cathode layer; and, (c) an emissive layer; wherein: (i) said emissive layer is interposed between said anode layer and said cathode layer; (ii) said emissive layer comprises an electroluminescent, semiconducting, organic material; (iii) said cathode layer comprises a layer of metal oxide having a thickness of from about 15 to about 200 xc3x85; and, (iv) said metal oxide is selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, lanthanide metal oxides, and mixtures thereof.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Diodes and particularly light emitting diodes (LEDs) fabricated with conjugated organic polymer layers have attracted attention due to their potential for use in display technology (Burroughs et al., 1990, Braun et al., 1991). Among the promising materials for use as active layers in polymer LEDs are poly(phenylene vinylene), PPV, and soluble derivatives of PPV such as poly(2-methoxy-5-(2xe2x80x2-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, a semiconducting polymer with an energy gap Eg of xcx9c2.1 eV. This material is described in more detail in Wudl et al., 1993a. Another material described as useful in this application is poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), BCHA-PPV, a semiconducting polymer with an energy gap Eg of xcx9c2.2 eV. This material is described in more detail in Wudl et al., 1993a, 1993b. Other suitable polymers include, for example, the poly(3-alkylthiophenes) as described by Braun et al., 1992 and related derivatives as described by Berggren et al., 1992; poly(para-phenylene) as described by Grem et al., 1992, and its soluble derivatives as described by Yang et al., 1993; and polyquinoline as described by Parker, 1994a. Blends of conjugated semiconducting polymers with non-conjugated host or carrier polymers are also useful as the active layer in polymer LEDs, as described by Zhang et al., 1994b. Also useful are blends comprising two or more conjugated polymers, as described by Yu et al., 1997. Generally, materials for use as active layers in polymer LEDs include semiconducting conjugated polymers, more specifically semiconducting conjugated polymers which exhibit photoluminescence, and still more specifically semiconducting conjugated polymers which exhibit photoluminescence and which are soluble and processible from solution into uniform thin films.
In the field of organic polymer-based LEDs, it is common to employ a relatively high work function metal as the anode, which serves to inject holes into the otherwise filled xcfx80-band of the semiconducting, electroluminescent polymer. Relatively low work function metals are preferred as the cathode material, which serves to inject electrons into the otherwise empty xcfx80*-band of the semiconducting, electroluminescent polymer. The holes which are injected at the anode and the electrons which are injected at the cathode recombine radiatively within the active layer and light is emitted. Proposed criteria for suitable electrodes are described in detail by Parker, 1994b.
Typical relatively high work function materials for use as anode materials include transparent conducting thin films of indium/tin-oxide (see, for example, Burroughs et al., 1990, Braun et al., 1991). Alternatively, thin.films of polyaniline in the conducting emeraldine salt form can be used (see, for example, Cao et al, 1997 Gustafsson et al., 1992., Yang-et al., 1994, 1995, and Yang, 1998). Thin films of indium/tin-oxide and thin films of polyaniline in the conducting emeraldine salt form are preferred because, as transparent electrodes, both permit the emitted light from the LED to radiate from the device in useful levels.
Typical relatively low work function metals which are suitable for use as cathode materials are metals such as calcium, magnesium, and barium. Alkali metals tend to be too mobile and act to dope the emissive layer (e.g. electroluminescent polymer), thereby causing shorts and unacceptably short device lifetimes. Alloys of these low work function metals, such as, for example, alloys of magnesium in silver and alloys of lithium in aluminum, are also known (see, for example, VanSlyke, 1991a, 1991b, Heeger et al., 1995). The thickness of the electron injection cathode layer typically ranges from about 200 to about 5000 xc3x85 (see, for example, VanSlyke, 1992, Friend et al., 1993, Nakano et al., 1994, Kido et al., 1995). A lower limit of about 200 to about 500 xc3x85 is required in order to form a continuous film (full coverage) for a cathode layer (see, for example, Holmes et al., 1996, Utsugi, 1998, Scott et al., 1996; Parker et al., 1994c). In addition to good coverage, thicker cathode layers were believed to provide self-encapsulation to keep oxygen and water vapor away from the active parts of the device.
Unfortunately, although the use of low work function electrodes is required for efficient injection of electrons from the cathode and for satisfactory device performance, low work function metals are typically unstable and readily react with oxygen and/or water vapor at room temperature and even more vigorously at elevated temperatures. Although alloying such low work function metals with more stable metals, such as, for example, aluminum or silver, has been used in attempts to improve environmental stability, the resulting cathodes remain unstable with respect to reaction with oxygen and/or water vapor during device fabrication and processing.
Despite improvements in the construction of polymer LEDs, a persistent problem has been rapid decay of the device efficiency (and light output) during stress, especially at elevated temperature. There is thus a need for low work function cathodes for use as electron-injecting contacts in organic (e.g., polymer) LEDs which have improved stability with respect to reaction with oxygen and water vapor especially at elevated temperature, and hence longer device lifetimes.
The alkali metals, alkaline earth metals and lanthanide metals are low work function metals. Although highly reactive (for example, with respect to oxygen and water vapor), they are utilized as cathodes in polymer or organic light-emitting diodes (LEDs) because they function as excellent electron-injecting contacts.
Applicants have discovered that cathodes comprising a thin layer of metal oxide (which metal oxide is selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, lanthanide metal oxides, and mixtures thereof) yield LEDs which offer comparable or better initial performance (e.g., brightness and efficiency), as well as comparable or extended operating lives, as compared to similar LEDs which employ conventional (e.g., metal) cathodes.
One aspect of the present invention pertains to light-emitting diodes (LEDs) comprising: (a) a hole-injecting anode layer; (b) an electron-injecting cathode layer; and, (c) an emissive layer; wherein: (i) said emissive layer is interposed between said anode layer and said cathode layer; (ii) said emissive layer comprises an electroluminescent, semiconducting, organic material; (iii) said cathode layer comprises a layer of metal oxide having a thickness of from about 5 to about 200 xc3x85; and, (iv) said metal oxide is selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, lanthanide metal oxides, and mixtures thereof.
In one embodiment, said metal oxide is selected from the group consisting of alkali metal oxides. In one embodiment, said metal oxide is selected from the group consisting of oxides of lithium, sodium, potassium, rubidium, and cesium. In one embodiment, said metal oxide is lithium oxide. In one embodiment, said metal oxide is selected from the group consisting of alkaline earth metal oxides. In one embodiment, said metal oxide is selected from the group consisting of oxides of magnesium, calcium, strontium, and barium. In one embodiment, said metal oxide is selected from the group consisting of oxides of magnesium and barium. In one embodiment, said metal oxide is selected from the group consisting of lanthanide metal oxides. In one embodiment, said metal oxide is selected from the group consisting of oxides of neodymium, samarium, and ytterbium.
In one embodiment, said layer of metal oxide has a thickness of from about 10 to about 100 xc3x85. In one embodiment, said layer of metal oxide has a thickness of from about 20 to about 60 xc3x85.
In one embodiment, said cathode layer further comprises a capping layer comprising aluminum, silver, or copper.
In one embodiment, said electroluminescent, semiconducting, organic material is an electroluminescent, semiconducting, organic polymer. In one embodiment, said electroluminescent, semiconducting, organic material is selected from the group consisting of: poly(p-phenylene vinylene)s, poly(arylene vinylene)s, poly(p-phenylene)s, poly(arylene)s, and polyquinolines. In one embodiment, said electroluminescent, semiconducting, organic material is poly(2-(3,7-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene).
In one embodiment, said electroluminescent, semiconducting, organic material is an electroluminescent, semiconducting, organic non-polymeric material.